Add a bit of backwards compatibility for specifying auth in the config

The doc example still shows the ability to pass auth as follows:

    <?php
    public $request = array(
        'method' => 'GET',
        'uri' => 'http://example.com',
        'auth' => array(
            'method' => 'Basic',
            'user' => null,
            'pass' => null
        ),
    );
    ?>

But this configuration fails due to restructuring of HttpSocket.  One could simply update the documentation, but it seems fairly straightforward to add a couple lines to maintain backwards compatibility.

I've simply added a check similar to the one that checks for 'user' and 'pass' in the uri component to check for this format of auth specification, and added tests to ensure it's working.

I understand if CakePHP doesn't want to be terribly concerned with backwards compatibility at this point, in which case the documentation should be updated.